This application claims priority of Japanese PCT Application No. PCT/JP00/04111 filed Jun. 22, 2000, the complete disclosure of which is hereby incorporated by reference.
The present invention relates to a vehicle rear wheel steering device, and in particular to a rear wheel steering device in a two rear axle type vehicle in which the rear part of the vehicle is supported by a rear-front axle and a rear-rear axle.
Conventionally, there has been widely used a truck with rear two axles as a truck having a high maximum carrying load since only a rear axle consisting of a single axle cannot bear a majority of a carrying load. In particular, with the use of a trunnion type suspension unit suspending rear two axles, no unbalance in load is present between a rear-front axle and a rear-rear axle, and accordingly, both axles can stably bear the carrying load.
In a truck having such a configuration that rear two axles are suspended by a trunnion type suspension unit, all load applied to the rear two axles are born at the middle point between the rear-front axle and the rear-rear axle, that is, at the attaching position of the trunnion bracket. Accordingly, the load is intensively applied to this single point in the longitudinal direction of the vehicle even with the configuration of the rear two axles, and accordingly, a high load has to be born at this position.
In such a truck having rear two axles, it may be considered that both rear two axles are shifted forward in order to reduce, in particular, the load exerted to front wheels having a single tire. That is, the load exerted to the front axle can be reduced by biasing the attaching position of the trunnion bracket forward. However, when both rear two axles are shifted forward, the overhanging part of the vehicle, afterward from the rear-rear axle becomes longer, and accordingly, there is raised such a problem that the rear end of the vehicle swings right and left during turning of the vehicle.
In order to allow the length of the overhanging part to fall in a predetermined range, and as well to reduce the load exerted to the front wheels, the rear front axle is shifted forward and to set the distance between the rear-front axle and the rear-rear axle to be longer. However, should the unsteerable rear-front axle largely biased forward from the rear-rear axle, rear-front wheels (tires) attached to both ends of the rear-front axle would eccentrically be worn during turning of the vehicle. As a result, there has been raised such a problem that the use life of the rear-front wheels (tires) is shortened.
In order to prevent eccentric abrasion of the rear-front wheels (tires), it may be consider to steer the rear-front wheels in response to steering of the front wheels. However, the rear-front wheels bear a high load in comparison with the front wheels, and accordingly, each of the rear-front wheels is usually composed of double wheels having two tires. Thus, a steering unit having a large-sized actuator which can exhibit a large steering force is required for steering the rear-front wheels. As a result, the steering unit becomes large-scale, and accordingly, there has been raised such a problem that the manufacturing cost for the vehicle is increased.
Further, when the rear-front wheels are steered sensitively in response to fine steering of the front wheels, the steering mechanism for the rear-front wheels is operated frequently, causing abrasion of the steering mechanism for the rear-front, as a result, the durability of the steering mechanisms deteriorates, thereby the reliability of the vehicle becomes lower.
Further, when the rear-front wheels are steered sensitively in response to fine steering of the front wheels, the rectilinear running ability of the vehicle is deteriorated during high speed running, and accordingly, the running stability of the vehicle is lowered during high speed running.
A first object of the present invention is to provide a rear-wheel steering device in which a substantial part of a carrying load is born by rear two axles so as to reduce the burden to front wheels, and further, the rear-front wheels can be steered so as to prevent eccentric abrasion of the rear- front wheel tires.
A second object of the present invention is to provide a rear wheel steering device which can steer the rear-front wheels with a relatively small-sized actuator so as to prevent deterioration of the durability of the rear-front wheels, which is caused by unnecessary steering, and further, which can enhance the rectilinear running ability during high speed running, thereby it is possible to improve the running stability.
The invention relating to claim 1, as shown in FIGS. 1 to 3 and FIG. 8, is an improvement in a rear wheel steering device in a vehicle in which a rear part of a vehicle body is supported by a rear-front axle 18 and a rear-rear axle 20.
The characteristic configuration of this invention comprises a rear-front wheel steering mechanism for steering right and left rear-front wheels 17, 17 which are rotatably mounted at opposite ends of a rear-front axle 18, an actuator 51 for driving the rear-front wheel steering mechanism so as to steer the rear-front wheels 17, and control means 69 for controlling the actuator 51, wherein the control means 69 stops the steering for the rear-front wheels 17 when a vehicle speed is approximately zero, but the control means 69 operates the actuator 51 when a difference between a theoretical steering angle of the rear-front wheels 17, relative to a steering angle of the front wheel 15, and an actual steering angle exceeds a predetermined value even though the vehicle speed is approximately zero.
It is noted that the control means 69 may be constituted so as to cause the actuator 51 to be inoperative when a transmission 25 is set at a neutral position. Further, the control means 69 may be constituted so as to cause the actuator 51 to be inoperative when a clutch is disengaged.
The invention relating to claim 4, as shown in FIGS. 1 to 3 and 8, is an improvement in a rear wheel steering device in a vehicle in which a rear part of a vehicle body is supported by a rear-front axle 18 and a rear-rear axle 20.
The characteristic configuration of this invention comprises a rear-front wheel steering mechanism for steering right and left rear-front wheels 17, 17 which are rotatably mounted at opposite ends of a rear-front axle 18, an actuator 51 for driving the rear-front wheel steering mechanism so as to steer the rear-front wheels 17, and control means 69 for controlling the actuator 51, wherein the control means 69 operates the actuator 51 in accordance with a steering angle of front wheels 15 so as to steer the front-rear wheels 17, and the control means 69 causes the actuator 51 to be inoperative so as to prevent the rear-front wheels 17 from being steered when the steering angle of the front wheels 15 is not more than a predetermined value.
In this arrangement, a ratio of the steering angle of the rear-front wheels 17 with respect to that of the front wheels 15 may be changed in accordance with a vehicle speed, the higher the vehicle speed, the smaller the ratio of the steering angle of the rear-front wheels 17. Further, the critical values in the range of the steering angel of the front wheels 15, in which the rear-front wheels 17 are prevented from being steered, may be changed in accordance with a vehicle speed.
The invention relating to claim 7, as shown in FIGS. 1 to 3 and 8, is an improvement in a rear wheel steering device in a vehicle having three axles, in which a front part of a vehicle body is supported by a front axle 16 while a rear part of the vehicle body is supported by a rear-front axle 18 and a rear-rear axle 20.
In the characteristic configuration of this invention, an inter-axle distance, B, between the rear-front axle 18 and the rear-rear axle 20 is set to be not less than 1,600 mm, and rear-front wheels 17 are mounted at the rear-front axle 18 so as to be steerable, and the rear-front wheels 17 are constituted so as to be steered during turning of the vehicle.
In this arrangement, the rear-front axle 18 may be a dead axle or a driven axle while the rear-rear axle 20 may be a drive axle. Further, both rear-front wheels 17 and rear-rear wheels 19 may be of double wheel type.
In a preferred embodiment of the present invention, the inter-axle distance, B, between the rear-front axle 18 and the rear-rear axle 20 for supporting the rear part of the vehicle body is set to be not less than 1,600 mm, and preferably set to be not less than 1,800 mm, and the rear-front axle 18 and the rear-rear axle 20 are arranged so that the inter-axle distance, B, between the rear-front axle 18 and the rear-rear axle 20 becomes not less than 20%, preferably not less than 25% with respect to an inter-axle distance, A, between the front axle 16 and the rear-rear axle 20. It is noted that should this rate is less than 20%, a load exerted to the front axle 16 could not be sufficiently reduced. Further, the rear-rear axle 20 is used as a drive axle having rear-rear wheels 19 which are attached to opposite ends of the drive axle so as to be unsteerable while the rear-front axle 20 is a driven axle having the rear-front wheels 17 which are attached to the opposite ends of the driven axle so as to be steerable, and further, the rear-front wheels 17 are constituted so as to be steered in accordance with a steering angle of front wheels 15 by means of an electronic control unit 69 and an actuator 51.
Steering for such rear-front wheels 17 under electronic control is not carried out when the vehicle speed is approximately zero, and accordingly, the rear-front wheels 17 can be steered by the actuator 51 even though the actuator 51 is relatively small-sized and has a low output power. However, even though the vehicle speed is approximately zero, the actuator 51 is operated when a difference between an ideal steering angle of the rear-front wheels 17, relative to that of the front wheels 15, and an actual steering angle is not less than a predetermined angle, and accordingly, the rear-front wheels 17 can be smoothly steered during a start of the vehicle.
Steering of the rear-front wheels 17 during running is carried out in association with a steering angle of the front wheels 15, and the ratio of the steering angle of the rear-front wheels 17 with respect to that of the front wheels 15 can be changed in accordance with a vehicle speed, that is, the higher the vehicle speed, the less the ratio of the steering angle of the rear-front wheels 17 relative to that of t he front wheels 15. When the steering angle of the front wheels 15 is not more than a predetermined value, the steering of the rear-front wheels 17 is stopped, that is, a dead zone is provided so that unnecessary steering of the rear-front wheels 17 is prevented. Further, the critical values of the steering angle of the front wheels 15 in the de ad zone where the steering of the rear-front wheels 17 is stopped, are changed in accordance with a vehicle speed so as to enhance the rectilinear running ability during high speed running, in order to enhance the running stability.